1 1 Green Chemistry Metrics David J.C. Constable 1.1 Introduction and General Considerations Green chemistry has recently been recognized as having achieved its twenty-fifth anniversary [1,2]. Across this span of time there has been a steady growth in green chemistry-related research and a deepening understanding of what green chemistry is and what it is not, although it is fair to say that it is still consistently debated. Since the earliest discussions about green chemistry, it is also fair to say that the field of chemistry has not embraced green chemistry nor seen the pur- suit of green chemistry for the good science and innovation opportunities that are inherent to it. Green chemistry is fundamentally how one thinks about chemistry and how one performs or practices chemistry. Interestingly and per- haps unfortunately, green chemistry has been seen by many chemists as some- thing akin to a social movement similar to other environmental movements [3]. A consequence of this perception is that green chemistry is sometimes seen as not being worthy of serious scientific consideration, in spite of the fact that the major proponents of green chemistry have always pointed to the fact that it is intended to spur innovation and promote the very best science while seeking to avoid or prevent human health and environmental problems. Green chemistry is not about environmental chemistry; to do the best green chemistry, one must however, understand environmental chemistry. It is also not about end-of-pipe environmental improvement, although this is often an important area to pursue. Part of the problem for this mistaken perception of green chemistry, especially when the term was beginning to be used, was a lack of precision or rigor in providing evidence when making claims about chemistry and labeling a new innovation as “environment-friendly” or “eco-friendly” or “green” or “greener.” The good news is that over the past 15 or so years, there has been considerable work on the part of many in the green chemistry and engineering community to develop methodologies and approaches to systematically and rigorously assess whether or not something is green or sustainable [4–23] (Curzons, A.D., Consta- ble, D.J.C., and Cunningham, V.L. (2002) Bond Economy: An Alternative Handbook of Green Chemistry Volume 11: Green Metrics, First Edition. Edited by David J. Constable and Concepción Jiménez-González. 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA. 2 1 Green Chemistry Metrics Approach to Synthetic Efficiency, Unpublished, GlaxoSmithKline.). Ironically, perhaps, despite all the work to develop metrics, there are still a large number of researchers and practitioners in chemistry publishing research in the top green chemistry and engineering journals who either ignore the use of metrics, claim that it is impossible to assess whether or not something is truly green or sustain- able, or use a single metric to justify calling their chemistry innovation “green.” This is truly unfortunate because much of what is published in the green chem- istry and engineering literature, if viewed from a multivariate metrics perspec- tive, is decidedly lacking in sufficient justification for a chemistry innovation to be called “green,”“greener,” sustainable, or more sustainable. It is worth taking a moment to discuss some general ideas about metrics before getting into the details of green chemistry and engineering metrics. Gen- erally speaking, it is commonly accepted that metrics must be clearly defined, simple to interpret and use, measurable, rely on objective determinations rather than subjective approaches, and should ultimately drive some kind of desired behavior or practice. The best way to use metrics is in a comparative sense; it is generally not very productive to engage in assessments from an absolute or abso- lutist frame of mind, particularly when focusing on sustainability. Rather, one should choose a frame or point of reference against which to apply a metric or make a comparison. In this way, one is able to say with some confidence that one particular outcome or impact is better or worse, greener or more sustainable than the alternatives. Absolutes tend to get in the way of people moving forward and making progress, and the lack of knowing something absolutely is frequently the reason given as to why nothing can or should be done. For example, “there are many possible approaches to metrics and who knows which ones are the best” is frequently offered as a reason for not applying metrics of any kind. This is a spurious argument at best. Another thing to consider is that one’s approach to metrics should be from a multivariate perspective. That is to say that the assessment of “green” or “sus- tainable” should be from multiple discreetly different kinds of measurements and adapted for a given context. Stated slightly differently, a single metric is insufficient to characterize a chemical, a type of chemistry, a process, or a prod- uct as being green, greener, or more sustainable. For example, just because a chemical transformation is done with a catalyst does not mean that it is as green as may be presumed. If, for example, that catalytic transformation is carried out using a platinum group element, and the catalyst is a homogeneous one (as opposed to a heterogeneous one), it is not green or sustainable from multiple perspectives. First, platinum group metals currently favored by many chemists, for example, iridium or platinum, are extremely rare, with relatively low abun- dance in the Earth’s crust. Second, the mining and subsequent extraction of these metals from ore, followed by the refining of those metals to separate the various elements that commonly occur in the ore is a highly mass and energy intensive process and results in significant environmental impacts from waste ore, spent extraction liquors, and so on. Finally, running a catalytic reaction homogeneously where the metal is not extracted from the spent mother liquors, 1.1 Introduction and General Considerations 3 it is likely for that metal to end up as a trace contaminant of incinerator slag, unlikely to be recycled, and most likely disposed of as hazardous waste. The last example about platinum group metals is also a good example of thinking about chemistry from a systems perspective, and/or using life cycle thinkingtomakeamorerealisticassessmentofwhetherornotsomethingis “green” or sustainable. Systems thinking is something that is more common in the biological sciences, where dependencies and connections between and among living systems are seen in the context of their ecological niche, the eco- system an organism inhabits, and the broader environment containing that eco- system. In other words, life is seen to exist as a complex web of interactions and interdependencies where perturbations to that system are felt in multiple parts of the system, although they may not be immediately noticeable. Systems think- ing is also more common in chemical engineering, where one is forced to see a particular unit operation in the context of the overall process, or one is thinking about mass and energy integration across a plant. In the latter instance, mapping the mass and energy inputs and outputs of every unit operation and thinking about how these might be better utilized to increase the overall mass and energy efficiency of the plant is now quite common and a well-developed practice. Among many chemists, however, research is focused on the immediate envi- ronment of two reactants in a round-bottom flask, along with some additional reagents, solvents, and catalysts. The idea that the choice of reactants, reagents, or solvents made in a laboratory for a given experiment has an impact on a broader system, the environment, or humans seems to be largely irrelevant to what may appear as the much more interesting consideration of whether or not two chemicals may react in a novel way. The previous discussion about platinum group metals is a great example of systems thinking and how few chemists are equipped to employ it. If they were, they might think twice about using an ele- ment like iridium as a homogeneous catalyst in a biomass conversion process. While its immediate benefit for catalytically converting lignin to a useful frame- work molecule, for example, may illustrate interesting chemistry that has not been previously done, using an extremely rare element that is dispersed as waste and effectively lost is a dubious application at best, and the approach is unlikely to be ever commercially applied. A familiarity with life cycle thinking and the even more desirable life cycle inventory/assessment methodology would help chemists in systems thinking, the idea of boundary conditions, and human or environmental impacts trading. Life cycle inventory/assessment in the context of green chemistry is covered in great detail in Chapter 4 as well as elsewhere [24–37].Theideaoftheboundaryin which an assessment is carried out that is associated with life cycle impact/ assessment is very important if one wants to perform a sustainability or green assessment. Where one draws the boundary for the assessment will likely make considerable differences in the outcome of the assessment. For example, just performing an environment, health and safety, or a life cycle impact category assessment (i.e., the ozone depleting potential, greenhouse gas equivalent, eutro- phication potential, etc.) of materials used in a particular reaction (boundary is 4 1 Green Chemistry Metrics limited) may be a good starting point, but it neglects the cumulative impacts associated with the materials throughout their life cycle. If one does actually do a fully burdened cradle-to-gate or cradle-to-cradle impact assessment, one will invariably be faced with many difficult questions. For example, which is more important – the cumulative greenhouse gas impact or the ozone-depleting impact, or the nonrenewable resource impact? Another aspect of metrics that is worth keeping in mind is that metrics should not be collected just for the sake of keeping metrics.